Engines may use a turbocharger to improve engine torque/power output density. The turbocharger may include a compressor and a turbine connected by a drive shaft, where the turbine is coupled to the exhaust manifold of the engine and the compressor is coupled to the intake manifold of the engine. The exhaust-driven turbine supplies energy to the compressor to increase the flow of compressed air into the engine. The use of a compressor allows a smaller displacement engine to provide as much power as a larger displacement engine, but with additional fuel economy benefits.
However, compressors are prone to surge and choke. For example, when an operator tips-out of an accelerator pedal, air flow decreases, leading to reduced forward flow through the compressor at high pressure ratio (PR), possibly leading to compressor surge. In another example, surge may be caused in part by high levels of cooled exhaust gas recirculation (EGR) which increase compressor pressure while decreasing mass flow through the compressor. Compressor surge can lead to NVH issues such as undesirable noise from the engine intake system.
Compressor choke may be encountered at high flows, when an increase in compressor speed gives a diminishing increase in the rate of flow. When the flow at any point in the compressor reaches the choke condition, no further flow rate increase is possible. This condition represents the maximum compressor volumetric flow rate as a function of the pressure ratio. Choke occurs when the air flow mass through the compressor cannot be increased for a given speed of the compressor. The flow rate into the compressor may be limited by the size of the compressor inlet, and when the flow at the inlet reaches sonic velocity, the flow may not be increased further. As one example, choke may occur when an operator tips-in from a part load or idle conditions to a high load condition, such as when going uphill with a load.
Various approaches have been developed to operate a compressor outside of both the surge and choke boundaries by reducing the air flow rate before surge occurs and increasing the air flow rate before choke occurs. One example approach includes the use of a passive casing treatment for a compressor. In one example, the passive casing treatment may include an immovable slot and/or ports that modify the air flow through the compressor. During low air mass flow conditions, the slot of the passive casing treatment may provide a path to recirculate partially pressurized air back to the compressor inlet. The recirculated air flowing through the compressor may enable the compressor to operate with lower air mass flow rate before surge occurs. During high air mass flow conditions, the slots and/or ports of the passive casing treatment may provide a path to short-circuit air flow through the compressor so that the compressor may operate with a higher air mass flow rate before choke occurs. It has been recognized that one drawback of passive casing treatment systems is that an effective location for a passive recirculation slot to prevent surge is different from an effective location for a passive recirculation slot to prevent choke.
Another example approach includes the use of an active casing treatment (ACT) for a compressor, such as shown by Sun et al. in U.S. Pat. No. 8,517,664. Therein, a turbocharger includes an active casing treatment, an impeller, a casing, and a diffuser. A controller adjusts a casing sleeve responsive to mass flow conditions relative to a threshold, or based on a pressure differential in the engine system, so that slots in the casing sleeve align with either a surge slot or a choke slot. Air is selectively flowed between the impeller and the compressor inlet responsive to the slot alignment.
However, the inventors herein have recognized potential issues with such an approach. As one example, adjusting an active casing treatment (ACT) responsive to current engine operating conditions is a reactionary control method that may sacrifice efficiency and performance by not adjusting compressor operation until a compressor is already at or near a surge or choke condition. In another example, this type of reactionary control method may result in a high frequency actuation of the casing sleeve to expose (e.g., open) the choke or surge slot, which may lead to flow pulsations, further reducing compressor efficiency and degrading performance.
In one example, the issues described above may be addressed by a method for a boosted engine, comprising: actuating a sleeve of a variable geometry compressor casing to a position selected based on each of a compressor pressure ratio and a mass flow through the compressor; and adjusting each of an EGR actuator and a boost actuator based on the selected position to maintain the compressor pressure ratio during the actuating. In this way, the disturbances associated with actuation of an active casing treatment may be compensated for such that a constant pressure flow during ACT actuation may be maintained, allowing compressor operation over a wider range of operating conditions.
As one example, a boosted engine may be configured with a turbocharger having a variable geometry compressor (VGC) and an exhaust turbine. The VGC includes an impeller surrounded by a casing and an active casing treatment. The casing includes a compressor inlet, an intake passage, a recirculation passage, a surge port, a choke port, and an actuatable sleeve having a bleed port. Responsive to compressor operation within a choke margin (that is, a threshold distance from a compressor choke limit), an engine controller may actuate the casing sleeve to a choke slot causing air to flow from the compressor inlet to the impeller via the choke port of the casing. Responsive to the actuation of the sleeve to the choke slot, high pressure EGR flow may be increased by increasing the opening of a high pressure EGR valve so as to maintain air flow and pressure ratio across the compressor despite the opening of the choke port. In addition, a boost actuator such as a waste-gate valve position or a blade angle of a variable geometry turbine may be adjusted to compensate for any disturbances arising from the actuation of the sleeve to the choke slot. In comparison, responsive to compressor operation within a surge margin (that is, a threshold distance from a compressor surge limit), the engine controller may actuate the casing sleeve to a surge slot causing air to flow from the impeller to the compressor inlet via the surge port of the casing. Responsive to the actuation of the sleeve to the surge slot, high pressure EGR flow may be decreased by decreasing the opening of the high pressure EGR valve so as to maintain air flow and pressure ratio across the compressor despite the opening of the surge port. In addition, the waste-gate valve position or variable geometry turbine (VGT) blade angle may be adjusted to compensate for any disturbances arising from the actuation of the sleeve to the surge slot.
Further, the controller may dynamically (e.g., in real-time) adjust each of the choke margin to the choke limit and the surge margin to the surge limit based on driver behavior, such as based on a frequency and degree of driver pedal application, as well as drive conditions, such as road grade and altitude. If the energy demand of driver pedal application is higher, such as may occur when the driver tends to drive aggressively, at least the surge margin may be increased so that the casing sleeve is engaged to the surge slot earlier and released from the surge slot later in a drive cycle.
In this way, driver behavior may be filtered to allow for a smooth engage/disengage profile of the ACT. In addition, ACT actuation frequency is reduced. By limiting ACT actuation to a threshold actuation frequency, flow pulsations and efficiency loss from the ACT actuation may be reduced. By adjusting EGR flow and boost actuator operation based on the ACT actuation, any flow pulsations or disturbances arising from the actuation can be better mitigated, improving overall compressor performance. By adjusting the choke and surge margins based on driver behavior as well as predicted travel conditions, greater surge and choke protection can be provided. Overall, compressor operation may be better optimized for both choke and surge conditions, and the operating range of the turbocharger may be extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.